Ground bushing



Aug. 19, 1969 E. WINSTON v GROUND BUSHING Filed July 15, 1966 INVENTOR EP/c' /4 //V670A f A'PTORNEY United States Patent US. Cl. 174-65 9 Claims ABSTRACT OF THE DISCLOSURE The invention contemplates a flanged insert of relatively hard metal threaded into tight permanent metaldisplacing engagement with a wall opening in an electrical-equipment case of relatively soft metal. The metaldisplacing binding electrically-grounding engagement occurs between a narrowly tapering or wedging outer surface on the flange, as it interferes with a right-cylindrical counterbore of the case opening. The axially outer end of the insert is also so formed as to define an axially projecting circumferential ridge for tight electrically conducting line-contact with the radial flange of a connector element fitted to the insert.

This invention relates to a threaded insert, of the variety used to provide electrical-connector access to and through the wall of a metal housing for electrical equipment.

In the past, some inserts of the character indicated have comprised bushings force-driven into permanent fitted position in the Wall of a junction box or other electrical-equipment case. Other such inserts have been externally threaded, and reliance has been placed on the security of attachment to the case, by deliberately forming the case aperture so that an interference fit with the threads is necessary. I have found these and other approaches to be unsatisfactory for certain electronic applications, as where the most reliable, weather-tight, vibration-resistant, electrically grounded relation must be established between the case and the threaded bushing.

It is, accordingly, an object of the invention to provide an improved insert of the character indicated.

It is a specific object to provide an improved insert of the character indicated, wherein electrical grounding contact is circumferentially uniform and continuous, for a range of tolerance variations in the formation of the receiving opening in the case, and particulary effective as an RF (radio-frequency) grounding element.

Another specific object is to meet the above objects with a structure which is inherently weather-sealed and highly resistant to other environmental conditions, such as mechanical vibration.

Still another specific object is to form such an improved insert with means inherently establishing a weather-sealing, good-grounding connection between the insert and the electrical fitting which is ultimately applied to the insert.

It is a general object to meet the foregoing objects with a structure which is inherently simple, easy to fabricate, and which in particular lends itself to precision massproduction installation, under controlled stressed conditions.

Other objects and various further features of novelty and invention will be pointed out or will occur to those skilled in the art from a reading of the following specification in conjunction with the accompanying drawings. In said drawings, which show, for illustrative purposes only, a preferred form of the invention:

FIG. 1 is an exploded view in elevation of parts incorporating features of the invention, certain parts being broken away and shown in longitudinal section;

FIG. 2 is an enlarged fragmentary longitudinal sectional view of an insert of the invention, in the process of assembly to the wall of a case;

FIG. 3 is a view similar to FIG. 2, upon assembly of the insert to the case; and

FIG. 4 is a view similar to FIG. 3, upon assembly of a connector fitting to the installed insert.

Briefly stated, the invention contemplates a flanged insert of relatively hard metal threaded into tight permanent metal-displacing engagement with a wall opening in an electrical-equipment case of relatively soft metal. The metal-displacing binding and electrically-grounding engagement occurs between a narrowly tapering or wedgin-g outer surface on the flange, as it interferes with a right-cylindrical counterbore of the case opening. The axially outer end of the insert is also so formed as to define an axially projecting circumferential ridge for tight electrically conducting line-contact with the radial flange of a connector element fitted to the insert.

Referring to the drawings, the invention is shown in application to a threaded bushing or insert 10 to be permanently secured to the tapped hole or opening 11 in the wall 12 of an electrical equipment case or box. The bore 11 is formed with a counterbore 13 at the outer end, having a right-cylindrical surface of diameter D exceeding the maximum diametral extent of the threads of bore 11. The case 12 is of relatively soft metal, such as cast aluminum.

The insert 10, on the other hand, is of relatively hard metal, such as stainless steel. It is provided with external threads 14 to engage the threads in bore 11, and it is formed at its axially outer end with a radially outwardly directed flange 15 having an outer surface 16 which is tapered for wedging interference engagement with the cylindrical counterbore 13. The entire outer surface 16 is of greater diameter than the maximum diameter of threads 14; also, the inner and outer diameters of surface 16 straddle the diameter of counterbore 13, regardless of tolerance variations. The divergence of taper surface 16 with respect to the insert axis 17 is designated a and is in the range of 9 to 15, being preferably 12 where the case metal is cast aluminum and the insert metal is stainless steel. For simplified assembly, i.e., for self-piloting initial alignment, the insert threads 14 extend outwardly from an axially inner cylindrical land 18, and for ease of fabrication, threads 14 and flange 15 are spaced by a similar land 19.

The insert bore 20 is shown threaded, for reception of the external threads 21 of a connector fitting 22. At the axially outer end of bore 20, i.e., beneath or within flange 15, a counterbore 23 is formed, tapering with essentially the slope of surface 16. Counterbore 23, throughout its tapering surface, exceeds the maximum diametral extent of the bore threads 20. Finally, the axially outer limit of insert 10 is defined by a hard, sharp ridge 25, established by intersection of counterbore taper 23 with a converging end-face taper 24. The convergence of face 24 is preferably at substantially a right angle (suggested at ,9) with respect to the inner and outer tapers 16-23, as viewed in a longitudinal section.

The assembly of insert 10 to threaded opening 11 is best illustrated in FIGS. 2 and 3. Rotary drive to insert 10 is imparted by a suitable tool (not shown) threaded to the insert bore 20. Driving torque is continuously applied after initial contact between taper surface 15 and counterbore 13, producing physical displacement of the softer metal by the harder metal as friction and local heat develop over the surface; rotary drive continues until a predetermined limiting torque is reached, as by using conventional torque-limited power wrench of the impact variety. This metal-displacing action will be seen to dispose of aluminum axide at the resulting tapered interface (see FIG. 3) and an optimum metallic-aluminum to metallic-stainless-steel bond and electric contact are achieved under circumferentially continuous controlled stress, symbolically suggested by crosses at the region 13. The ultimate fit of the insert places the outer end taper 24 virtually in flush alignment with the outer flat wall surface of the case 12, leaving the ridge projection 25 extending sufliciently axially outwardly to permit assured driven contact with the radial flange surface 26 of the connector fitting 22.

Connector 22 is shown with an outwardly facing groove 27 in which an elastomeric seal ring, such as O-ring 28, is located, between threads 21 and radial flange surface 26. It will be appreciated that, on take-up of threads 21- 20, ring 28 is compressed by counterbore taper 23, thus providing a secondary seal, but that the biting action of ridge 25 into the radial connector surface 26 establishes a primary sealing, securing and grounding action.

It will be seen that I have described an improved grounding or other electrical-connection insert of the character indicated, achieving the foregoing objects, and lending itself to accurately reproducible effectiveness with mass-production techniques using power tools, such as an impact wrench, for assembly purposes.

.A superior R-F ground is assuredly established with my insert, even when sealant is applied to eliminate any possible airflow through the installed insert-to-case connection, under the most extreme diurnal temperature fluctuations. For example, although not shown in the drawings, it will be appreciated that, upon hard-driven take-up of the thread engagement at 11-14, a spiral passage for possible air or other gas flow will be established by normal clearance behind the tightened threads; this clearance is preferably completely filled by a suitable sealant, which may be an anaerobic or other hardenable flexible liquid, applied initially to the external insert threads 14. As the insert is driven into home position, excess sealant (suggested at 30 in FIG. 4) is accommodated within counterbore 13, without contacting or in any way interfering with the direct and intimate metal-to-metal grounding contact at 16-13.

While I have described the invention in detail for the preferred form shown, it will be understood that modification may be made without departing from the scope of the invention.

I claim:

1. In combination, a case of relatively soft metal and having a threaded bore through a wall thereof, said bore having a counterbore at the axial outer end, said counterbore being initially formed with a right-cylindrical peripheral wall of diameter exceeding the maximum diameter of the threaded bore; an insert sleeve of relatively hard metal having a cylindrical body with external threads engaged to the threads of said bore, said sleeve being internally threaded and formed at its axially outer end with a radially outwardly extending flange, the radially outer surface of said flange tapering from an axially inner end at a diameter less than that of said counterbore and diverging to an axially outer end at a diameter greater than that of said counterbore, said tapered outer surface being so engaged to said counterbore as to displace substantial adjacent counterbore metal and to cause said counterbore to assume the tapered contour of said flange, the axially outer end of said flange being conically tapered, the axially outer end of the sleeve bore being tapered, the sleeve-bore taper and the axially outer end taper being divergent in opposite directions and intersection in a relatively sharp circumferentially continuous circular ridge which constitutes the axially outer limit of said sleeve; and a threaded connector engaged to the sleeve-bore threads, said connector including an outwardly directed radial flange surface in circumferentially continuous engagement with said ridge.

2. The combination of claim 1, in which said case is of cast aluminum.

3. The combination of claim 1, in which said sleeve is of stainless steel.

4. The combination of claim 1, in which said connector has a circumferentially extending radially outwardly facing groove between said radial surface and the threads of said connector, and an elastomeric ring in said groove and compressed by and beneath the adjacent sleeve-bore taper.

5. The combination of claim 1, in which the axial depth of the counterbore is such in relation to the interference fit of said flange thereto that an axially extending annular clearance is defined between the axially inner end of said flange and the axially inner end of the counterbore, whereby excess sealant applied to said threads, and squeezed-out as the insert is taken-up into the threaded bore, may be accommodated in said clearance without impairing the direct and intimate metal-to-metal grounding contact at the interference fit of said flange.

6. In combination, a case of relatively soft metal and having a threaded bore through a wall thereof, said bore having a counterbore at the axial outer end, said counterbore being initially formed with a cylindrical peripheral wall of diameter exceeding the maximum diameter of the threaded bore; an insert sleeve of relatively hard metal having a cylindrical body with external threads engaged to the threads of said bore, said sleeve being formed at its axially outer end with a radially outwardly extending flange, the radially outer surface of said flange tapering from an axially inner end at a diameter less than that of said counterbore and diverging to an axially outer end at a diameter greater than that of said counterbore, said tapered outer surface being so engaged to said counterbore as to displace substantial adjacent counterbore metal and to cause said counterbore to assume the tapered contour of said flange upon threaded take-up of said sleeve in the bore of said case, said sleeve having a bore including internal threads, the axially outer end of said flange being conically tapered to an axially outer end at intersection with the sleeve bore, thereby defining a circumferentially continuous circular ridge which constitutes the axially outer limit of said sleeve; and a threaded connector engaged to the sleeve-bore threads, said connector including an outwardly directed radial flange in circumferentially continuous engagement with said ridge upon threaded take-up of said connector in the bore of said sleeve.

7. The combination of claim 6, in which the taper of the radially outer surface of said flange is at an angle of 9 to 15 degrees divergence with respect to the sleeve axis.

8. The insert of claim 7, in which the divergence of said taper issubstantially 12 degrees.

9. In combination, a case of relatively soft metal and having a threaded bore through a wall thereof, said bore having a counterbore at the axial outer end, said counterbore being initially formed with a cylindrical peripheral wall of diameter exceeding the maximum diameter of the threaded bore; an insert sleeve of relatively hard metal having a cylindrical body with external threads engaged to the threads of said bore, said sleeve being formed at its axially outer end with a radially outwardly extending flange, the radially outer surface of said flange tapering from an axially inner end at a diameter less than that of said counterbore and diverging to an axially outer end at a diameter greater than that of said counterbore, said tapered outer surface being so engaged to said counterbore as to displace substantial adjacent counterbore metal and to cause said counterbore to assume the tapered contour of said flange upon axially-stressed threaded take-up of said sleeve in the bore of said case, said sleeve having a bore including internal threads, the axially outer end of said flange being conically tapered to an axially outer end at intersection with the sleeve bore, thereby defining a circumferentially continuous circular ridge which constitutes the axially outer limit of said sleeve, whereby said ridge 5 presents a circumferentially continuous electrical grounding contact for the radial flange of a flanged electrical connector when threaded into axially-stressed relation with said sleeve.

References Cited UNITED STATES PATENTS 6 FOREIGN PATENTS 500,583 3/1954 Canada.

THOMAS P. CALLAGHAN, Primary Examiner US. Cl. X.R. 

